Blender painting brushes shortcut selection polygons 2.82. Coloring a character with vertex-group in draw mode
Your viewport has a button there that I highlighted that turns on Face Selection Masking, and then you can select the polygons you want to limit your paint to.

I see your project is very high poly, seemingly in dyntopo sculpt so you are using Vertex Paint. If you were to first make a copy of the mesh, you might try remeshing to get a quad version at low enough resolution that you could use multiresolution modifier to resculpt lost detail but then afford to uv unwrap and use actual texture paint mode and make use of layers of images.
Edit: Here is a shot of where you can convert a copy of the dyntopo sculpt into remeshed quads in sculpt mode, but this is only when dyntopo is turned off.

This one shows what kind of result I went with prior to adding a multiresolution modifier and resculpting close details.